1. Field of the Invention
The invention relates to a method of producing homogeneously doped n-type Si monocrystals and somewhat more particularly to a method of producing homogeneously doped n-type Si monocrystals having a specific resistivity greater than about 30 ohm .times. cm by thermal neutron radiation.
2. Prior Art
Si crystal bodies, such as rods, are generally doped after the precipitation or deposition of solid Si onto a heated mandrel or rod-shaped carrier member with the aid of thermal and/or pyrolytic decomposition of gaseous Si compounds. In such a process, dopants are intermixed with a gaseous Si compound and decompose at the carrier member so as to be dispersed within the forming Si body. The Si rods or bodies so produced are polycrystalline and must be converted into a monocrystalline state by subsequent zone melting processes. During such zone melting, the concentration of the dopant within the Si rod often changes in an uncontrollable manner and considerably higher dopant concentrations must be provided in the polycrystalline rods in order to attain a desired dopant concentration in the final monocrystalline rods, especially in instances where a plurality of zone melting processes are utilized. However, such processes are time-consuming and inexact. Further, the devices available for carrying out such processes are only marginally satisfactory and are extremely expensive.
Other methods of producing doped Si crystal bodies are also known, for example, from German Offenlegungsschrift Nos. 1,544,276 and 2,020,182. These prior art references suggest that select dopants be converted to a gaseous state and fed, with the aid of a carrier gas flux, to a molten Si material positioned within an evacuated reaction chamber so that the gaseous dopant is directly blown or carried, for example, into the molten zone of a Si rod undergoing crucible-free zone melt processing. Boron and/or phosphorous compounds which are easily handleable and easily vaporized are generally the dopants utilized with this method. The dosage or concentration of dopants supplied to the molten Si is regulated via valves. However, a great drawback to such methods is that the valves used to control the dopant dosage do not operate in the necessarily exact manner to provide reproducible results. In addition, these methods provide a more or less non-homogeneous distribution of dopants within the finished rod after zone melting. The semiconductor components produced from such inexactly doped Si rods cannot obtain their optimum characteristic properties since the fluctuation of dopants during the growth process of the monocrystalline rods becomes noticeable during the zone melting processes by forming facets and uneven temperature distributions in the melt; in other words, the fluctuations of dopant cause noticeable radial and axial inhomogeneities in the specific resistivity of such a rod; for example, during the occurrence of "striations" which are inhomogeneities of material concentration, fluctuations occur nearly periodically in the crystal.
M. Tanenbaum et al., "Preparation of Uniform Resistivity n-Type Silicon by Nuclear Transmutation", 108 Journal of Electrochemical Society, No. 2, pages 171-176 (February 1961) suggests that Si crystals having n-type conductivity may be produced by radiation of thermal neutrons on pure Si crystals. In this process, the natural isotope Si.sup.30, which is present in pure Si crystals is transmuted into the unstable isotope Si.sup.31 by the capture of a thermal neutron and emission of .gamma. radiation. The unstable Si.sup.31 isotope decays by .beta..sup.- emission with a 2.62 hr. half-life into the stable P.sup.31 isotope. However, pure starting crystals which are required for this process are costly.